Satellite-based positioning systems include constellations of earth orbiting satellites that constantly transmit orbit information and ranging signals to receivers. An example of a satellite-based positioning system is the Global Positioning System (GPS), which includes a constellation of earth orbiting satellites, also referred to as GPS satellites, satellite vehicles, or space vehicles (SVs). The GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to the earth. The satellite signal information is received by GPS receivers which can be in portable or mobile units, or in fixed positions on base stations and/or servers.
The GPS receiver uses the satellite signal information to calculate the receiver's precise location. Generally the GPS receiver compares the time GPS signals or satellite signals were transmitted by a satellite with the time of receipt of that signal at the receiver. This time difference between satellite signal reception and transmission provides the receiver with information as to the range of the receiver from the transmitting satellite. Using pseudo-range measurements (pseudo because the range information is offset by an amount proportional to the offset between GPS satellite clock and receiver clock) from a number of additional satellites, the receiver can determine its position. The GPS receiver uses received signals from at least four satellites to calculate three-dimensional position (latitude, longitude, and altitude), or at least three satellites to calculate two-dimensional position (if altitude is known).
As GPS technology becomes more economical and compact it is becoming ever more common in consumer applications. For example, GPS systems are used for navigation in general aviation and commercial aircraft as well as by professional and recreational boaters. Other popular consumer uses of GPS include use in automobile navigation systems, construction equipment, and farm machinery as well as use by hikers, mountain bikers, and skiers, to name a few. Further, many location-based services are now available, such as asset tracking, turn-by-turn routing, and friend finding. Because GPS technology has so many consumer applications, it is finding increased popularity as an additional application hosted by a variety of portable electronic devices like personal digital assistants (PDAs), cellular telephones, and personal computers (PCs), to name a few. The popularity of GPS technology with consumers has resulted in an increased reliance on the position information provided to the consumer by GPS which, in turn, has resulted in a desire for GPS systems that provide reliable position information even when the GPS system is operating under less-than-ideal conditions.
The GPS satellite signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not get through most solid objects such as buildings and mountains. As a result, GPS receivers are generally usable everywhere except where it is impossible to receive an adequate satellite signal such as inside some buildings, in caves and other subterranean locations, and underwater. A GPS receiver, when determining position information, typically relies on information from the satellite signal, the absence of which makes position determination impossible. This satellite signal information includes a pseudorandom code along with ephemeris and almanac data to the receivers. The pseudorandom code is a code that identifies the satellite that is transmitting the corresponding signal and also helps the receiver to make ranging measurements. The almanac data tells the GPS receiver where each GPS satellite of the constellation should be at any time over a wide time interval that spans a few days or weeks. The ephemeris data does the same thing but much more accurately though over a much shorter time interval of a few hours.
The broadcast ephemeris (BE) data, which is continuously transmitted by each satellite, contains important information about the orbit of the satellite, and time of validity of this orbit information. In particular, the broadcast ephemeris data of a GPS satellite predicts the satellite's state over a future interval of approximately four hours. Broadcast ephemeris enables predictions of satellite position, velocity, clock bias, and clock drift. More particularly, the BE data describe a Keplerian element ellipse with additional corrections that then allow the satellite's position to be calculated in an Earth-centered, Earth-fixed (ECEF) set of rectangular coordinates at any time during the period of validity of the broadcast ephemeris data. Typically, the broadcast ephemeris data is essential for determining a position.
Because the broadcast ephemeris data is only valid for a four hour interval and is essential for position determination, a GPS receiver is required to collect new broadcast ephemeris data at such time as the receiver needs to compute the satellite state when the validity time for the previously-collected broadcast ephemeris data has expired. Broadcast ephemeris that is still valid may be referred to as “current” broadcast ephemeris. Current broadcast ephemeris data can be collected either as direct broadcast from a GPS satellite or re-transmitted from a server. However, there are situations under which it is not possible to collect new broadcast ephemeris data from GPS satellites or from a server. Examples of situations in which new broadcast ephemeris data cannot be collected include: a low signal strength of the satellite signals can prevent decoding/demodulating of the ephemeris data from the received satellite signal, the client can be out of coverage range of the server, and/or the server can be unavailable for a number of reasons, to name a few. When new broadcast ephemeris data is not available, the GPS receiver is typically unable to provide position information.
To address the need in the art for GPS receivers operable to determine satellite ephemeris without reception of current broadcast ephemeris, commonly-assigned U.S. Pat. No. 7,142,157 (the '157 patent) discloses a server that receives or collects historical state data of satellites for a satellite-based positioning system and numerically integrates the historical state data to provide predictions of satellite trajectories based upon the historical state data. These predicted satellite states may also be denoted as satellite ephemeris.
It will be appreciated by those of ordinary skill in the arts that the term “ephemeris” is then being used in its strict sense. Although it is conventional in the GPS arts to refer to the transmission of Kepler parameters by the GPS satellites as “broadcast ephemeris,” Kepler parameters are not “true” satellite ephemeris but instead are parameters derived from satellite ephemeris. Because the reference to the conventional transmission of Kepler parameters from GPS satellites as “broadcast ephemeris” is a firmly-entrenched practice in the GPS arts, the results from a numerical integration of historical state data may be referred to as “predicted satellite states” or “extended ephemeris” to avoid confusion with parameters such as Kepler parameters that are merely derived from satellite ephemeris.
Having calculated the predicted satellite states, the server may transmit these states to client GPS-enabled devices. These client devices may then calculate current satellite ephemeris based upon the predicted satellite states. A time period spanned by the predicted satellite states depends upon the desired accuracy. For example, if +/−40 meter accuracy is acceptable, the predicted satellite states may correspond to every 15 minutes over a seven day period for all the satellites in the GPS constellation. To determine satellite states at a current time within this seven day period, the client device need then merely interpolate the relevant predicted satellite states about the current time. In this fashion, the client device needs relatively little processing power to determine current satellite states. However, considerable bandwidth and storage facilities must be dedicated to the transmission and storage of so many predicted satellite states.
Thus, the '157 patent discloses alternative embodiments in which the server does not generate predicted satellite states but rather parameters derived from these predicted satellite states such as Kepler parameters. A client device receives the Kepler parameters from the server and may thus predict satellite trajectories using the Kepler parameters. In this fashion, bandwidth demands for the transmission between the server and the client devices are reduced. However, the amount of data necessary for transmission may still strain bandwidth-limited connections between the server and the client GPS devices. For example, a full set of Kepler parameters valid for a four hour period for one satellite may comprise 422 bits. Six such sets of Kepler parameters to cover a 24 hour period for a satellite thus equals 422*6=2532 bits (or approximately 317 bytes). A complete day's transmission of Kepler parameters for the complete GPS constellation (twenty-seven space vehicles) thus requires 317*27=8449 bytes, which may prove to be too large to transmit over, for example, a bandwidth-limited wireless connection.
Accordingly, there is a need in the art for extended ephemeris solutions that lessen the bandwidth demands between a server of extended ephemeris data and its GPS client devices.